1. Field of the Invention
The present invention relates to a communications system and in particular, but not exclusively, to one having two domains or segments.
2. Description of the Related Art
Known office based communication systems usually operate with fixed line telephone units within the office linked via an internal switch board or PBX (private branch exchange). Such fixed line systems are able to provide relatively high voice quality although users are not able to move for the duration of a call.
The advent of wireless cellular telecommunication technologies, an example of which is the GSM standard (Global System for Mobile communications) means that wireless systems can provide at least the equivalent voice quality as compared to fixed line systems. Wireless cellular systems also have the advantage that the user can move.
WIO “wireless intranet office” is a proprietary communication system which is being developed by the present applicants. This WIO system introduces the concept of utilizing mobile terminals, such as conventional GSM mobile stations, in an office environment. The system makes use of a known concept called internet telephony or voice-over-IP. (IP stands for internet protocol.)
Voice-over-IP allows audio, video and data information to be transmitted over the existing IP-based local or wide area networks or the Internet. The technology thus provides for convergence and integration of the communication of three different data types over the same network.
Prior to the introduction of voice-over-IP, offices often operated three separate networks for the transmission of audio, video and data information. Fixed line telephone systems coupled to an in-house PBX provided voice communication. An office based local area network (LAN) or intranet (i.e. a packet switched internal network) having computer terminals linked via network cards and under the control of a server station provided for the transmission of “conventional” computer data. Video cameras linked to monitors via a fixed line or wireless link provided for video communications. With voice-over-IP, audio, video and data information can be transmitted simultaneously using the same packet-switched network or IP-router throughout the office environment and beyond the confines of the office.
In order to provide for such media convergence, voice-over-IP often uses a specific ITU (International Telecommunication Union) standard protocol to control the information flow over the intranet. One common standard protocol used in voice-over-IP systems, and the one used in the WIO system is termed H.323. H.323 is an ITU standard for multimedia communications (for example voice, video and data) and allows multimedia streaming over conventional packet-switched networks. The protocol provides for call control, multi media management and bandwidth management for both point-to-point (two end users in communication) and multipoint (three or more end users in communication) conferences.
The H.323 protocol is network, platform and application independent allowing any H.323 compliant terminal to operate in conjunction with any other H.323 terminal.
The H.323 standard defines the use of three further command and control protocols. H.245 is for call control and is responsible for control messages governing the operation of the H.323 terminal including capability exchanges, commands and indications. Q.931 is for call signalling and is used to set up a connection between two terminals. The RAS (Registrations, Admissions and Status) signalling function governs registration, admission and bandwidth functions between endpoints and gatekeepers. Gatekeepers will be discussed in more detail hereinafter.
For a H.323 based communication system, the standard defines four major components: terminals; gateways; gatekeepers; and multipoint control units (MCU).
Terminals are the user end-points on the network and can be a fixed telephone, a fax unit or a computer terminal. All H.323 compliant terminals must support voice communications and optionally video and data communications.
Gateways connect H.323 networks to other networks or protocols. For an entirely internal communications network, i.e. one with no external call facility, gateways are not required.
Gatekeepers are the control centers of the voice-over-IP network. It is under the control of a gatekeeper that most transactions (communication between two terminals) are established. The primary functions of the gatekeeper are bandwidth management call control controlling the number of simultaneous H.323 connections and controlling the amount of bandwidth these calls consume. An H.323 “zone” is defined as a collection of all terminals, gateways and multipoint-control units (MCU) which are managed by a single gatekeeper. Multipoint control units (MCU) support communications between three or more terminals. The MCU comprises a multipoint controller (MC) which performs H.245 negotiations between all terminals to determine common audio and video processing capabilities, and a multipoint processor (MP) which routes audio, video and data streams between terminals.
The conventional voice-over-IP system described hereinbefore normally utilizes standard fixed-line telephone systems which are subject to the disadvantages of a lack of mobility.
The WIO concept takes voice-over-IP further in that it provides for the use of conventional mobile terminals, such as GSM mobile stations, within the voice-over-IP system. To provide for such mobile communications within an intra-office communication network, the WIO system combines known voice-over-IP with the use of conventional wireless terminals within the voice-over-IP system. Those telephone units may be GSM mobile stations. This WIO concept may be in the context of a intra-office communication network.
Thus, in the WIO system intra-office calls to mobile stations are routed through the office intranet and external office calls are routed conventionally through the GSM network. Such a system provides most or all of the features supported by the mobile station and the GSM network such as telephone directories, short messaging, multiparty services, data calls, call barring, call forwarding etc. WIO thus provides for integrated voice, video and data communications by interfacing an H.323 based voice-over-IP network with a GSM mobile network.
The WIO system is a cellular network similar to the conventional GSM network and is divided into H.323 zones as discussed hereinbefore. One H.323 zone may comprise a number of a GSM cell. Two or more H.323 zones may be contained with an administrative domain.
Security problems can arise between the office and operator domains of the network. The operator domains of the network are those which are administered by an operator and will typically include the mobile communication supporting components. The office domain typically includes the office intranet and is maintained by the corporation.
It has been recognised by the inventors that H.323 traffic would be rejected by network protection measures, thus preventing cross domain calls to take place.
A second problem with the proposed WIO system arises when there is a call between a fixed H.323 terminal and a mobile station. Currently, if a mobile station moves from being supported by one network element to being supported by another network element, difficulties can arise. In particular, the fixed terminal would need to break its connection with the network element which previously supported the mobile station and establish a new connection with the network element which now supports the mobile station. This is a cumbersome process and data can be lost unnecessarily.